Synthesis, structure and diffusion pathways of fast lithium-ion conductors in the polymorphs α- and β-Li8SnP4

The increasing demand for a high-performance and low-cost battery technology promotes the search for Li+-conducting materials. Recently, phosphidotetrelates and -aluminates were introduced as an innovative class of phosphide-based Li+-conducting materials featuring ionic conductivities of up to 3 mS cm(-1) at ambient temperature. In order to get a deeper understanding in structure-property relationship of lithium ion conductors closely related structures that differ in their ionic conductivity are of special interest. Here, we report on the two polymorphs alpha- and beta-Li8SnP4, which show ionic conductivities of up to 0.7 mS cm(-1) and low activation energies E-A of about 28 kJ mol(-1) (0.29 eV) at 298 K. The structures of the two phases are determined by single crystal X-ray and powder neutron diffraction experiments at different temperatures, and their significantly different ionic conductivities allow for a detailed insight into the structure-property relationship. The investigations are completed by Li-6, P-31 and Sn-119 solid state magic angle spinning NMR, temperature-dependent Li-7 NMR experiments and electrochemical impedance spectroscopy. Negative nuclear density maps reconstructed from experimental structure factors were analyzed by the maximum entropy method (MEM) and the one-particle-potential (OPP) formalism. Distinct Li+ migration pathways including divergent activation barriers have been identified, which allow to interpret the different conductivities of the two modifications. The importance of partially occupied octahedral sites in the beta-phase is ascertained to cause considerably lower energy barriers to adjacent tetrahedral voids, which promote the higher conductivity in comparison to the alpha-phase. The title compounds complete the series of three phosphidotetrelates (alpha-)Li8SiP4, alpha-Li8GeP4 and beta-Li8GeP4, and allow a detailed investigation of the structure-property relationships for further tailoring of the material properties.